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Superheated steam and cast iron

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Ian Porthouse04/04/2011 11:49:31
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I have seen several references to superheated steam damaging cast iron.
 
Can any one explain how superheated steam damages cast iron fittings and cylinders and if this is dependant upon the degree of superheating?
 
I wonder if anyone in this fraternity has suffered any issues with cast iron and superheated steam and how one decides whether or not to include superheating in a boiler?
 
 
 
 
Lawrie Alush-Jaggs04/04/2011 12:30:37
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Hi Ian
 
I may off the track here but I seem to remember from reading a book from Babcock and Wilocox about twenty five years ago that there are several problems.
Cast Iron has excellent compressive strength (Victorian England would have fallen down without it) but very low elasticity (I think that is the correct term in the circumstances), a property which is diminished as the temperature goes up. In other words the material gets weaker as the presure and temperature goes up.
In the same book or maybe another, it mentioned the use of superheated steam in Naval vessels and quoted someting like 700 degrees F for 2,300 PSI.
Structurally cast iron is like cement rather than wood in that cement is lots of particles with inclusions - sand whereas wood has a grain.
Steel is more homogeneous than cast iron and better able to withstand bursting.
I'm not sure much of this makes sense but I hope it gives a direction for further research.
Ian Porthouse04/04/2011 13:41:17
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Thank you Lawrie,
 
Your explanation makes perfect sense.
 
700 degree F (371 Celsus) would appear to be achievable in a model boiler with superheater but I am not sure about 2,300 PSI.
 
It just seems strange that certain kit manufacturers produce cast iron cylinder castings and i.c. engine blocks, liners and cyinder heads may also be cast iron.
 
I would have thought that combustion temperatures in an i.c. engine would pass 700 degrees F ( but far less pressure).
 
The warnings always seem to specifically relate to superheated steam. I still wonder if the use of superheated steam on a model cast iron cylinder would cause damage.
John Olsen04/04/2011 21:11:17
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I seem to recall reading that the Germans made hydrogen for their Zeppelins during WW1 by passing steam over red hot iron. The iron grabs the oxygen, giving a mixture of hydrogen and steam. It is of course relatively easy to remove any remaining steam by allowing it to condense.
 
Presumably as this process goes on the iron is eroded away into rust.
 
regards
John
Lawrie Alush-Jaggs06/04/2011 00:27:25
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Hi Ian
 
I think John's answer goes a lot further to explaining the problem.
I mentioned 2,300 PSI for superheated steam though it was not in the context of model engineering, rather what was being done on US navy vessels in the late fifties and early sixties for turbines.
Personally I think that 2,300 PSI for models is out of the question too. I am sure you can't scale the piping down far enough for safety and your strange new laws certainly wouldn't allow it.

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